The invention relates to a method and a device for separating heavier from lighter parts of aqueous slurries by means of centrifugal force effects.
The invention more particularly relates to a cleaning of liquid slurries having a solid particle fraction below a certain dimension, i.e. to a subsequent cleaning of slurries which have already been subjected to precleaning for removal of coarse particles by screens or the like.
In separation by means of centrifugal separators or hydrocyclones a precleaned slurry is introduced at high velocity into a separating chamber to cause an intensive rotating laminary field of flow to form in the latter so that the heavier parts of the slurry are forced by centrifugal force effects to an outer diameter path, whilst the lighter parts of the slurry preferably collect in the vicinity of the longitudinal centerline of the separating chamber. In a known centrifugal separator (U.S. Pat. No. 2,996,187) the pressure gradient necessary for the flow of the slurry between the inlet and outlet of the separating chamber is produced by a suction transport rotor means provided downstream of the outlet of the separating chamber. The pressure gradient between the inlet and outlet is thus determined by the suction force of the suction transport rotor means and this suction force is in turn defined by the liquid column existing on the suction side so that by means of the suction transport rotor means a pressure gradient of only less than 1 bar can be created. The prior centrifugal separator can thus be used only for suspensions in which a sufficient separation effect is attained even at relatively low rotational speeds of the slurry. To treat parts of aqueous slurries less easy to separate, higher rotational speeds are needed in the separating chamber to generate correspondingly high centrifugal forces. This would necessitate a pressure gradient of a few bars which cannot be produced by known centrifugal separators so that in general several small dimensioned centrifugal separators would need to be arranged in series to arrive at a desired separation rate. This makes the procurement and operating costs of the separation plant substantially more expensive and increases its maintenance frequency. In addition, the thruput of small-dimensioned centrifugal separators is relatively low so that the application of systems equipped therewith is restricted to specific cases of application.
One problem especially involved is to effectively separate floatational particulate material from aqueous slurries by means of centrifugal force effects. It has been proposed (U.S. Pat. No. 4,397,741) for floatational separation to additionally introduce a gas into the slurry circulating in a separating chamber to produce bubbles of gas to which the separated heavier fractions of the slurry tend to adhere due to interfacial effects. The gas bubbles form, so-to-speak, buoyancy bodies so that the heavier fractions not only collect preferably in the vicinity of the longitudinal centerline of the separating chamber but can also be drawn off against the gravitational effect. The effectiveness of this known device is, however, low since the efficiency of separation is based solely on a tangential introduction of the slurry into the separating chamber, i.e. means of boosting the pressure gradient being totally absent and, in addition to this the gas bubbles achievable by the known means have too large a dimension.